The invention disclosed in this patent is a process for making electroluminescent devices with light-emitting polymers by screen printing. The process allows creation of patterns of varying size that can illuminate with varying degrees of brightness, either all at once or at different times to create an animated display.
In the late 1970""s, pioneering work showed that conjugated polymers, those systems consisting of alternating single and double bonds, could conduct charges and be effectively utilized as both semiconductors and conductors. The charge transport mechanism in polymers is due to the weak overlap of p-orbitals along the backbone of conjugated chains. Charge carriers that are introduced by injection or doping can be delocalized when traveling in these orbitals, or xcfx80 bonds, allowing for the effective conduction of electricity. Because polymers can be made semiconducting, they exhibit the similar properties to inorganic semiconductors, such as photoluminescence, electroluminescence, and photoconductivity. In electroluminescence, injected charged carriers of opposite sign (i.e. electrons and holes) can recombine to emit radiative light. In the radiative recombination process, the electron drops from a higher energy orbital down to the lower energy orbital emitting a photon with energy equal to the difference in the upper and lower energy levels. For most conjugated polymers, this difference in energy levels results in light emission in the visible energy spectrum. As such, conjugated polymers can be used to make light emitting devices that emit in the blue, green and red. The most efficient light emitting structure is normally a diode, or LED, since this enables efficient balanced injection of the charge carriers.
Although there was early optimism that conduction in polymers would lead to new technologies, it was not until 1990 and the discovery of efficient electroluminescence at low voltages in thin films of conjugated polymer materials, and shortly after in liquid soluble conjugated polymers, that the promise of semiconducting polymer electronics, and LEDs, started to be widely embraced. This research demonstrated the importance of using thin films to achieve high current densities, and subsequently high light output, at low voltages. Such thin films were needed in order to overcome the low mobilities of conjugated organics that is caused by the inherent disorder and the weak overlap of the xcfx80-orbitals. The electron and hole mobilities have recently been measured in a conjugated polymer as a function of temperature with the result that both electron and holes undergo space charge limited behavior with a current density Jxcx9cxcexc(T,V) V2/L3, where the mobility xcexc(T,V) is exponentially dependent on field and temperature, V is the applied voltage and L is the length of the conduction path through the polymer. This type of transport appears now to be widely seen in conjugated polymer semiconductors and emphasizes that the current is dramatically reduced with increased length of the conduction path. For example, a factor of 100 increase in conduction path length can result in a minimum six order of magnitude decrease in current density.
Once the need for thin film materials was realized, polymer semiconductors offered promise of a complete paradigm shift in the manufacture of semiconducting devices since they enable inexpensive liquid-based processing under atmospheric conditions rather than expensive high temperature and vacuum-based processing. This promise motivated significant progress in polymer material development, purity and stability over the last decade. The realization that the incorporation of charge transporting layers into the polymer device could greatly improved device efficiency and stability further aided the rapid development of the science and technology. Now, a decade after the initial discovery of electroluminescence in polymers, polymer light emitting diodes are competitive in efficiency and stability to many inorganic-based devices and are now on the brink of commercialization. However, despite this apparent success, the initial promise, that of inexpensive liquid-based processing, has yet to fulfilled.
This invention aims to fulfill the of light emitting polymer devices promise by describing a method to inexpensively manufacture liquid processible polymer-based thin film light emitting devices using a screen printing based manufacturing process.
Although large area applications such as LEP wallpaper to replace incandescent and fluorescent wall lamps have been suggested, the only practical (i.e., reasonably cost-effective) methods for applying the necessary thin, uniform thickness light-emitting polymer layers to large areas are spin coating, ink jet printing and screen printing.
Spin coating, while capable of achieving thin, uniform thickness layers, does not allow layers to be deposited as patterns independent of substrate shape, a critical aspect of this Invention. Furthermore, spin coating is not suited for surfaces much larger than 14xe2x80x3xc3x9714xe2x80x3 and is relatively slow, in terms of number of layers produced over a period of time.
Ink jet printing of LEP""s has been demonstrated for relatively small displays and could conceptually be applied to larger area patterned devices.
Screen printing of organic semiconductors and LEP xe2x80x9clight conversion layersxe2x80x9d has been described but not for printing a LEP emissive layer, again a critical aspect of this invention.